In recent years there has been growing interest in the use of conjugated, semiconducting polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies up to 8%.
The conjugated polymer serves as the main absorber of the solar energy, therefore a low band gap is a basic requirement of the ideal polymer design to absorb the maximum of the solar spectrum.
A commonly used strategy to narrow the band gap of conjugated polymers is to utilize an alternating copolymer consisting of both electron rich donor units and electron deficient acceptor units within the polymer backbone. An acceptor unit that is known in prior art and has shown good photovoltaic performances when used in copolymers is 2,1,3-benzothiadiazole (BTZ) (see J. Chen, Y. Cao, Acc. Chem. Res., 2009, 42 (11), 1709):

However, these polymers can have limited solubility in commonly used organic solvents, which can inhibit their suitability for device manufacturing methods based on solution processing). Therefore, the presence of additional solubilising functionality on the BTZ unit is desirable to expand the range of suitable processing solvents and their solid loading within these solvents.
More recently polymers containing 5,6-bis(octyloxy)-benzo[2,1,3]thiadiazole ((OR)2BTZ) units have been prepared, like for example copolymers with fluorene having the following structure:

These polymers show improved solubility due to the flexible alkoxy side chains (see J. Bouffard, T. M. Swager, Macromolecules, 2008, 41(15), 5559).
Additionally, where the (OR)2BTZ unit is flanked by two thiophene units, like in the copolymers shown below, the polymer retains the planar conformation of the back bone which is required to achieve the narrow band gaps and good charge carrier mobility required for OPV applications (see R. Qin, W. Li, C. Li, C. Du, C. Veit, H.-F. Schleiermacher, M. Andersson, Z. Bo, Z. Liu, O. Inganas, U. Wuerfel, F. Zhang, J. Am. Chem. Soc., 2009, 131, 14612; M. Helgesen, S. A. Gevorgyan, F. C. Krebs, R. A. J. Janssen, Chem. Mater., 2009, 21(19), 4669; W. Li, R. Qin, Y. Zhou, M. Andersson, F. Li, C. Zhang, B. Li, Z. Liu, Z. Bo, F. Zhang, Polymer, 2010, 51, 3031):

However, these polymers suffer from open circuit potentials (Voc) in OPV bulk-heterojunction devices which are not attaining the theoretical maximum of ˜1.15 V in a polymer/PCBM device (see J. C. Bijleveld, R. A. M. Verstrijden, M. M. Wienk, R. A. J. Janssen, Applied Physics Letters, 2010, 97, 073304).
Therefore, there is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processability, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, and do not suffer from open circuit potentials (Voc) in OPV bulk-heterojunction devices, or do so to a lower extent than polymers from prior art.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processability, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing conjugated polymers containing BTZ units that are substituted in 5- and 6-position by ester groups to give benzo[2,1,3]thiadiazole-5,6-dicarboxylic acid bis-ester:
wherein E is CO—O or O—CO and R1 and R2 are carbyl groups like for example alkyl or aryl.
It was found that conjugated polymers based on this unit show good processability and high solubility in organic solvents, and are thus especially suitable for large scale production using solution processing methods. At the same time, they show a low bandgap, high charge carrier mobility and high oxidative stability and are promising materials for organic electronic OE devices, especially for OPV devices. Also, the addition of two electron withdrawing ester groups onto the BTZ acceptor unit deepens the HOMO level in order to achieve a higher open circuit potential (Voc) in an OPV bulk-heterojunction device versus a device containing a polymer based upon BTZ or (OR)2BTZ while maintaining the same band-gap.